The production of near photographic quality images using electrophotographic imaging technology is highly desirable. It is even more desirable to produce such images on substrates that render the print with the look and feel of a typical photographic print produced with silver halide imaging technology, such as the degree and uniformity of glossiness, stiffness and opacity, and high resolution and sharpness with corresponding low grain appearance. The advantages to producing photographic quality images on such substrates using digital electrophotography include improved environmental friendliness, ease of use, and versatility for customizing images, such as when text and images are combined.
U.S. Pat. No. 5,846,637 describes a coated xerographic photographic paper comprised of (1) a cellulosic substrate; (2) a first antistatic coating layer in contact with one surface of the substrate; (3) a second toner receiving coating on the top of the antistatic layer, and comprised of a mixture of a binder polymer, a toner spreading agent, a lightfastness inducing agent, a biocide, and a filler; and (4) a third traction controlling coating in contact with the back side of the substrate comprised of a mixture of a polymer with a glass transition temperature of from between about −50° C. to about 50° C., an antistatic agent, a lightfastness agent, a biocide and a pigment. This paper provides for the third layer on the backside of the substrate to receive toner, but this is not sufficient for ensuring high image quality should the image be created on this third layer instead of the second layer on the other surface of the substrate.
European Patent Application 1,336,901 A1 describes an electrophotographic image receiving sheet with a toner image receiving layer containing a release agent and formed on a support sheet for use in a fixing belt type electrophotography. The support used in the examples had a paper base with polyethylene layers on either side, where the image side is glossy and the backside has a matte finish. No provision is made for receiving the toner image on the backside.
US Patent Application 2003/0082354 A1 discloses an image receiving sheet for electrophotography comprising a base paper and a toner image receiving layer comprising a thermoplastic resin and less than 40 percent by mass based on the thermoplastic resin, of a reinforcing filler pigment. The thermoplastic layer is infiltrated to a depth of 1 to 50 percent of the thickness of the base paper. It is desirable that the toner image receiving layer is substantially free of any pigment or filler in order to prevent blister formation and roughening of the toner image. The resin used for toner image receiving layer is preferably applied as a coating solution, the resins being soluble in water or dispersible in water and the solution's viscosity is preferred to be in the range of 10-300 mPa·sec. Similarly, US Patent application 2003/0082473 A1 discloses use of a coating liquid whose solution viscosity is preferred to be in the range of 20-500 mPa·sec.
US Patent application 2003/0037176 A1 discloses a electrophotographic transfer sheet that comprises a substrate having an image receiving layer that contains a thermoplastic resin as a main component, which has a melt viscosity at 120° C. of about 200 to 2,000 Pa·sec. This patent application discloses that if viscosity of the thermoplastic resin exceeds 2,000 Pa·sec, then burying of the color toner image receiving layer becomes insufficient and relief of the color toner image is formed on the surface which results in deterioration of gloss uniformity. The patent application also discloses coating methods like reverse roll coater, bar coater, curtain coater, die slot coater or gravure coater for creating the toner image receiving layer. The structure of the electrophotographic transfer sheet disclosed in this patent application has the toner image receiving layer only on one side.
US Patent application 2004/0058176 A1 discloses a electrophotographic image receiving sheet where the toner receiver layer is coated on an polyethylene layer coated on a base. Though a whole host of polymers and methods for creating the toner image receiving layer have been listed, this patent application does not teach what are the necessary properties of a resin that satisfy a process like extrusion coating of resins as well as adhesion to toner. The patent application claims that the thermoplastic resin in the toner image receiving layer is a self dispersing water dispersible polyester resin emulsion that satisfies the following properties: number average molecular weight (Mn)=5000, molecular weight distribution (ratio of weight average molecular weight/number average molecular weight)≦4, glass transition temperature (Tg) in the range of 40° C.-100° C. and volume average particle diameter in the range of 20 nm-200 nm. Another claim made by the patent application is the toner image receiving layer may also contain a polyolefin resin and this layer may be extrusion coated.
U.S. Pat. No. 6,217,708 discloses a full color transfer paper for electrophotography, which does not have a toner image receiving layer coated on it. This method has a shortcoming since it results in photographs or images that show mottle of the paper and other paper defects.
US Patent Application 2003/0175484 A1 discloses the creation of an image receiving sheet that has excellent gloss and has high offset resistance during a fixing step at a high temperature under high pressure. This is achieved by using a polyester resin containing at least 10% based on the molar number of polyhydric alcohol components of bisphenol A as a polyhydric alcohol component; and said polyester resin has an intrinsic viscosity (IV) of 0.3-0.7. This patent application does not discuss or claim about the branching of the polyester, neither does it discuss or claim the properties that enable extrusion coating.
US Patent Application 2003/0235683 A1 discloses an electrophotographic image receiving sheet comprising a support and a toner image receiving layer containing a thermoplastic resin and a pigment disposed on the surface of the support wherein the surface of the support has a glossiness of 25 percent or more at 75° and a pigment content less than 40 percent by mass based on the mass of the thermoplastic resin. In this case also it is desirable that the toner image receiving layer be substantially free of any pigment or filler in order to prevent blister formation. Toner particle size also plays a key role in determining image quality in electrophotography, smaller particles generally yielding better image quality. However, as the particles get smaller, the physics of the forces holding the particles to the photoconductor changes drastically, needing new methods to effectively transfer them from the photoconductor to the receiver. Photographic quality prints can be produced with this process if very small toner particles are used. The drawback with small particles is the difficulty in transferring them onto plain paper. One solution to this problem is explained in U.S. Pat. No. 4,968,578, where the surface of the receiver sheets are coated with a thermoplastic layer.
There exists a need for improved paper for electrophotographic printing that can provide high gloss, where differential gloss, image relief, and residual surface fuser oil are minimized and toner adhesion is maximized. Further it is desirable that such prints be fingerprint and spill resistant. Still further, customers perceive product quality in terms of stiffness for a photo quality print. Therefore there exists a need for creating media for electrophotographic printing of high stiffness for a given a caliper of the base. There also exists a need for creating low cost media for electrophotographic printing that can be created by polymer melt extrusion coating toner receiver (in prior art might be known as toner image receiving) layers.